In reply to zimpara:
If in 'pretend-instructor' mode helping with the Uni club (although I do have SPA) or similar, then cowstail and sling, grigri and twistlock carabiner, spare belay device and carabiner for teaching, two more twistlock carabiners, 2 prussiks that live on the back of my harness. Not that I would ever do this indoors unless it was an emergency or every member of wall staff suddenly injured themselves, but two spare screwgates and at least one prussik is the minimum you need to easily take over a loaded belay (one to back everything up with a knot/clove hitch, one for the prussik, although it's been a while since I last did it on training). Cowstail is useful for rapidly attaching yourself to things (duh). I once had a belayer holding a climber on a top rope where the belayer's harness split into two parts (the leg loops detached from the belay loop due to incorrect assembly) so they were holding the weight on just the waist loop. I just put the climber on belay as well and took it over, but it has subsequently been pointed out to me I could have just clipped my cowstail into the belay carabiner...
My prussiks live quite happily girth-hitched onto the rear leap of my harness (as already stated, putting anything else there is less than ideal) - why would I ever take them off? The odd spare screwgate is useful for attaching people to sandbags; you might even see me with a quickdraw and a sling or two if I am teaching someone leading! Finally my (trad) cowstail lives on my harness (I have a different one for sport climbing which I don't mind getting scratched up by bolts). I have used it indoors once (cheekily) threading the screwgate at the top of a route that I had lead up and wanted to put a top rope on but the screwgate had seized shut!
You do see people with some really unnecessary stuff on their harness, but prussiks, spare belay device (actually useful indoors!) and gloves are all quite reasonable things to have indoors...